you have right triangles and you basically use cosine to determine the length and then you could take that length and dived the full side of the paper and from that if you get whatever you wanted great. e.g. cos(78.75degrees)… (45+22.5+11.25) is .195 very close to .2 or 1/5.

[…] Well, Wonko of the Setting the Crease blog is here to the rescue! The blog’s author, Peter Whitehouse, was kind enough to post on his site a trick for folding paper into thirds, fifths and sevenths. Thank you! Check out his how-to post. […]